1. Field of the Invention
This invention relates to the field of self luminous light sources employing radioactive gas to activate phosphors deposited on surfaces of the light source, and in particular, to an elongated tubular light source charged with radioactive tritium.
2. Description of the Prior Art
Use of a radioactive gas and a phosphor coating responsive to the emissions of the gas, is known in the art. Such light sources generally take the form of simple glass tubes enclosing the radioactive gas in a cylindrical space. In connection with self-luminous light sources for watch dials and the like, rectangular or other enclosing shapes are employed. In any event, the conventional teachings of the art are to take a simple, sealed glass body having a phosphor coating on the internal surfaces thereof, and to charge the body with the radioactive gas. Particle emissions incident to radioactive decay of the gas within the body activate the phosphors on the inner surface of the external shell, producing light emissions by the phosphors.
Competing concerns such as safety on the one hand, and maximizing brightness and efficiency of the light source on the other hand, require the designer to make a number of choices in the configuration of a light source. A larger enclosed volume results in a larger active surface area, producing increased light. In order to further increase brightness, it has sometimes been found necessary to employ a radioactive gas of a type in which particles released during radioactive decay are emitted at relatively high energy, for example, krypton-85 (.sub.36 Kr.sup.85). Gas of this variety will cause photon emissions in phosphors at a distance on the range of tens of centimeters from the decaying atoms, however, such relatively-powerful emissions are not healthful for humans.
Safer levels of particle emission energy are obtained with use of tritium (.sub.1 H.sup.3) as the radioactive gas. Particles emitted by decaying tritium will activate phosphors within a range of millimeters from the decaying atoms. Accordingly, tritium is a preferred source of phosphor-activating emissions in light sources intended for use in proximity with persons. Unfortunately, such low energy particle emissions are also only able to produce a relatively weak level of light emission in the phosphors.
Production of any radioactive gas is a relatively expensive procedure. It is sometimes the case that the expense of mechanical construction, packaging and the like is small relative to the cost of radioactive gas used in self-luminous light sources. Use of a larger volume of gas and the resulting additional phosphor surface area will, up to a point, proportionately increase the total brightness of the light source. There is a limit, however, to gains in brightness per unit of radioactive gas achievable from using increased amounts of radioactive gas.
As the gas space becomes larger, more of the emitted particles are absorbed by neighboring atoms in the gas, and never reach the phosphor coating to produce light. With respect to tritium, for example, a glass tube having a phosphor coating on its inner surfaces and a diameter greater than several millimeters, i.e., the transmission range of tritium, will be of lower total brightness per unit of tritium (i.e., a lower efficiency) than a group of tubes each having a diameter within the transmission range and enclosing the same total amount of gas. In other words, with the single large diameter tube, emitted particles which happen to be directed radially inward are unlikely to ever reach the phosphors on the far side of the tube. These particles will be absorbed in the gas itself, and will not help produce light.
The relatively low energy of tritium emissions frequently makes tritium unattractive as a phosphor-activating element in a large light source. The low power emission capabilities of tritium means that increasing the diameter of the light source tube in order to increase surface area in fact makes an inefficient use of the tritium. In this respect efficiency is the total light emitted per unit of tritium.
U.S. Pat. No. 3,038,271--MacHutchin et al teaches a self-luminous sign in which a plurality of glass tubes of relatively small diameter are used to activate phosphor coatings over the area of the sign. If the diameter of each of the tubes is kept small, namely within the transmission range of tritium, the disclosed sign can be expected to be relatively efficient in production of light, that is, achieving a reasonable total brightness per unit of tritium, and therefore per unit of cost. Use of a plurality of separate closed glass tubes causes other problems, such as difficulty in production, handling, mounting and the like.
U.S. Pat. No. 3,566,125--Linhart, Jr., et al teaches a light source having a particular contour for the gas-holding space. Light emission is said to be improved by a parabolic facing surface on the phosphor-bearing body, which is enclosed within the light source. It is believed that the increase in luminosity of the Linhart device is due to the increase in phosphor-bearing surface area of a curved area over a flatter one. Linhart's respective embodiments include a number of arrangements in which the transmission range of radioactive emission is clearly exceeded, particularly as to emissions directed toward the rear of the parabolic surface of the phosphor-mounting body.
In the embodiment of FIG. 6, Linhart uses a collimating lens having a convex rear surface. The convex lens has a contour at least partly complementing the parabolic, phosphor-coated surface. Such restriction on the depth of the gas-enclosing space should be a relatively efficient use of radioactive gas. The construction has a number of drawbacks. The efficiency is achieved at expense of a need for multiple parts of dissimilar materials, and the need to connect the parts in a seal which will be impermeable to tritium. Tritium is, of course, a form of hydrogen, which is prone to difficulties with leakage and will diffuse directly through many materials.
U.S. Pat. No. 3,005,102--MacHutchin et al teaches simple gas-enclosing phosphor-coated tubes, but also discloses one embodiment in which a flashlight bulb is simulated using a gas-enclosing plenum of relatively-restricted depth. Reference may be made to MacHutchin's FIG. 3, in which the gas-charged plenum is laid over a hollow glass bulb, with expected increase in efficiency. MacHutchin U.S. Pat. No. 3,005,102, like Linhart Jr., appears to teach an arrangement restricting the depth of the gas space to a distance approaching the transmission range of the radioactive gas. Both patents, however, teach a plurality of dissimilar parts in complex constructions. The constructions, using complex geometrical shapes, and requiring gas-tight junctions, will certainly be difficult and expensive to manufacture. Increased manufacturing expenses may increase the product cost to an extent that safety and gas conservation gains are outweighed. Moreover, a number of usual junction-making materials are simply not feasible due to diffusion and loss of radioactive tritium through the seals.
U.S. Pat. No. 3,176,132--Muller teaches a refinement of the usual glass tube. A central tube, holding a souce of radioactive emissions, is mounted within a casing tube, and a plurality of coaxial phosphor-bearing tubes, or a spirally wound sheet of phosphor-bearing material, is disposed between the central axial radioactive tube and the casing. This construction is said to be useful to confine the radioactive emissions. While emissions may be confined, such a construction merely aggravates the difficulty with the low transmission range of tritium. Not only will the transmission range be possibly exceeded between the central source of radioactive emissions and the peripheral phosphors, but intermediate layers of phosphor-bearing material, phosphors and gas will themselves absorb emissions. Moreover, photon emissions from the excited inner phosphors must pass through multiple surrounding layers to reach the casing, in order to be released from the lamp as useful light. Accordingly, although Muller teaches a structure including coaxial tubes, the teachings emphasize safety over efficiency, and are more appropriate for high energy particle emitting gases and the like.
The present invention employs a central body and an external casing, the body and casing together defining a gas-enclosing space of restricted width around the device. The inward-facing walls of the enclosed space are preferably all coated with phosphors. The invention therefore conserves gas by not exceeding the transmission range of the gas, for example tritium. Inasmuch as the device is preferably formed by a pair of coaxial glass tubes, sealed to form an annular glass-bounded area, an integral glass body results in which no possibility of leakage or diffusion loss is presented.
Apart from radioactive self-luminous devices, in connection with electric discharge devices and chemically-operated self-luminous lamps, a number of coaxial tube constructions are known. In electric discharge devices, outer tubes are structured and intended as optical filters or for mechanical protection, and are not arranged to form a confined space for a radioactive gas. On the contrary, the operative gas and the electric discharge elements are almost invariably mounted in the central axial space. There is therefore no particular requirement of a complete enclosure around the inner tube. In addition, there is no difficulty with any safety consequences of radioactive emissions and no need for spacing of elements because the most dangerous emission expected from the light source (and the most often blocked via a shield) is ultraviolet radiation.
U.S. Pat. No. 3,358,167--Shanks teaches a jacketed electric discharge lamp in which an outer casing physically protects the operative electric discharge light source mounted along the axis. A resilient plug having a central circular opening for receiving the light source, and an annular groove for receiving the casing is disclosed.
U.S. Pat. No. 2,080,919--Ihln et al teaches a spring-like form of resilient spacer in an electric discharge device. The space between the light source and the casing is evacuated, to decrease heat loss by conduction/convection.
A third category of interest is light sources powered by chemical reaction. Unlike either electric discharge devices or radioactive light sources, chemically self-luminous devices employ sealed containers of reagents within an external casing. The containers frequently are tubular, and means are provided to break or otherwise open the containers and thereby mix the reagents. These devices seldom have any direct connection between inner and outer tubes.
The present invention involves a coaxial tube arrangement particularly adapted for self-luminous radioactive light sources. An optimum width gas enclosure is produced by a relatively inexpensive and easy to manufacture construction. The light source as so constructed can be further mounted in a casing with resilent shock-absorbing means and/or provided with a mounting as desired for a given use.